Just thought I'd share fork oil findings. When I bought my 1200 GT in Feb 2020, on the first ride the left fork seal blew, no drama, replaced plenty of seals before hence, ordered genuine oil and dust seals and 7.5w Motul fork oil as that's the oil weight the manual suggested. That was 30,000 miles ago, I've always thought that the front end was harsh over anything but a perfectly surfaced road, unable to absorb the bumps, especially bumps in quick succession, not what I expected the advanced (at the time) electronic suspension would be like. Over that 30k 4 year period I'd been reading that fork oil weights are not as they seem. A 7.5w oil from one manufacturer is not necessarily the same as a 7.5w from another, some of you may know this but I didn't. Seems that the recommended oil in the manual, apart from being very expensive and difficult to get hold of, is a lighter oil than it states. A few other guys, on a FB Multi site, had drilled down the many figures/stats of fork oils and came to the conclusion that Rock Oil SVi Synthesis 3.5w was the nearest comparable oil. Last weekend I stripped it down to address the head stock/steering bearings which were self centering and notchy, they were passed their best for sure! Bottom race was badly indented, probably should have replaced them 15k ago really. At the same time I rebuilt the forks with new seals and bushes, bushes were actually ok but, I'd got the parts so replaced them anyway, also replaced the oil with Rock 3.5w. Did a few test rides this weekend and the difference is night and day, so much more compliant and soaks up the imperfections in the road surface with ease, front end is now composed instead of skittish. Only regret is that I suffered 30k miles as it was! I would imagine that there are quite a few Skyhook equipped bikes riding round with the wrong oil in the forks, hope this helps some of them.
Good post Fork oil is ostensibly rated the same wa engine oil is, which is all to do with how it operates across a vast temperature range (even though it’s a single grade not a multi grade). However, the oil in the front forks doesn’t experience such temperatures as in an engine and what’s more important is it’s viscosity at a certain temperature ie the temperature your bike forks mostly operate at which is different in different climates. What’s of more importance is how easily or not the oil can be squeezed through the valves. This is what’s known as its kinematic viscosity and is measured in what’s known as centistokes. The easiest way to think of a centistoke is how many grams of oil can be squeezed through 1 square millimetre of an orifice (ie the damping valves inside the fork) per second. There’s a useful article here which compares many of the different fork oils out there. https://motoiservices.com/en/suspension-oil-equivalence-chart/
West Cork Paul - I saw a guy demonstrating that, as simple as a cup with a 1mm hole drilled in the bottom, he was timing how long each manufacturers oil took to drain the cup of 100ml of oil, the difference in time took between the same weight oil from different manufacturers could be considerable.
A simple 'useful' reaction on your post isn't justice enough, thank you! This thread is very helpful as I'll be servicing my forks this winter, and it'll be their first service since assembly in 2021!
